1. Field of the Invention
This invention relates in general to passive infrared motion detectors, and in particular to a passive infrared motion detector with a certain configuration of sensing elements together with an optical collection system that results in a detection pattern with no diverging insensitive zones in the volume of interest.
2. Description of the Prior Art
Passive infrared motion detectors generally use an optical collection system and multiple sensing elements of alternating polarity, or of same polarity but summed after alternately inverting and not inverting the signals from the sensing elements, to create a detection pattern in the volume of interest. Inherent in the detection patterns are insensitive zones in which the detector cannot sense motion. Most manufacturers currently use a configuration of sensing elements whereby each sensing element is separated from adjacent sensing elements by a gap approximately equal to the width of one sensing element. However, this popular configuration necessarily results in diverging insensitive zones as illustrated by the following discussion.
Referring to FIG. 1 and FIG. 2, the basic sensing element configuration of much of the prior art comprises an arrangement of pairs of oppositely poled sensing elements 16a and 16b (collectively referred to as sensing elements 16), constructed of a material that produces a usable signal when illuminated by infrared radiation (the most common range of infrared radiation used by motion detectors is between approximately 8 and 14 micrometers). In most of the prior art, each sensing element has a width w and is separated from other adjacent sensing elements by a gap roughly equal to width w. Typically, located in front of the sensing elements 16 is a lens array 13 with lens centers 18. Lens array 13 directs infrared radiation onto the sensing elements 16.
This arrangement of lens array and sensing elements results in a detection pattern with sensitive zones 20a and 20b (collectively referred to as sensitive zones 20) and diverging insensitive zones 22a and 22b (collectively referred to as insensitive zones 22). Sensitive zones 20a are the projection in space, through lenslet optical centers 18, of the sensing element 16a. Similarly, sensitive zones 20b are the projection in space, through lenslet optical centers 18, of the sensing element 16b. Insensitive zones 22 are areas from which radiation is not directed onto any of the sensing elements 16 (or depending on the geometry of the particular motion detector, areas in which positive and negative sensitive areas overlap and result in cancellation). Sensing elements 16 cannot detect motion in insensitive zones 22.
The problem that plagues the typical motion detectors of the prior art is that the insensitive zones 22 diverge, thereby getting larger with increasing distance from the lens array 13. This problem impedes detection of small movements at distances far from the detector. Manufacturers have unsuccessfully attempted to solve the problem of diverging insensitive zones in different ways.
One unsatisfactory solution which has been previously attempted is to juxtapose the two sensing elements. This configuration removes the diverging insensitive zones due to the gap between the sensing elements, but does not necessarily result in a uniform detection pattern. Further, this solution is unacceptable because it is accompanied by technical problems such as: electrical leakage between the elements; electrical breakdown from the high fields required for poling the elements; and thermal conduction between the elements which causes the energy focused on one element to diffuse to the element of opposite polarity, thereby reducing the signal. In an attempt to circumvent the above technical problems, some manufacturers have attempted leaving a space smaller than the width of each element between the elements. Although this technique somewhat alleviates the technical problems associated with juxtaposing the elements, it does not eliminate these technical problems and it reintroduces the problem of diverging insensitive zones.
Manufacturers and users are currently having to make and use passive infrared motion detectors with diverging insensitive zones.